Optical
fiber (or "fiber optic") refers to the medium and the technology associated
with the transmission of information as light pulses along a glass or plastic
wire or fiber. Optical fiber carries much more information than conventional copper
wire and is in general not subject to electromagnetic interference and the need
to retransmit signals. Most telephone company long-distance lines are now of optical
fiber. Transmission on optical fiber wire requires
repeaters at distance intervals. The glass fiber requires more protection within
an outer cable than copper. For these reasons and because the installation of
any new wiring is labor-intensive, few communities yet have optical fiber wires
or cables from the phone company's branch office to local customers (known as
local loops). Optical fiber consists of a core,
cladding, and a protective outer coating, which guide light along the core by
total internal reflection. The core, and the higher-refractive-index cladding,
are typically made of high-quality silica glass, though they can both be made
of plastic as well.

An optical fiber can break if bent too sharply. Due to the
microscopic precision required to align the fiber cores, connecting two optical
fibers, whether done by fusion splicing or mechanical splicing, requires special
skills and interconnection technology.Two main
categories of optical fiber used in fiber optic communications are multi-mode
optical fiber and single-mode optical fiber. Multimode fiber has a larger core
allowing less precise, cheaper transmitters and receivers to connect to it as
well as cheaper connectors.